The incident happened at Pant Nagar village in Magadh Medical College police station in Gaya district, police said.
The deceased were identified as Soni Kumari (20) and Annu Kumari (3).
"The accused Nikku Kumar attacked his wife Soni and daughter Annu with a sharp-edged weapon. Both died and he was held when taking the bodies in a sack on a bicycle to dump in the Phalgu river," Gaya Town Deputy Superintendent of Police (DSP) Alok Kumar Singh said.
The DSP said Kumar had written his mobile numbers on a piece of paper and kept it with the bodies in the sack so that later those who found them could inform him.
This was an attempt to evade suspicion about him, the DSP said.
